Introduction

Anaerobic digestion (AD) is a process in which complex organic substrate is degraded to yield methane containing biogas (Kleerebezem, Joosse, Rozendal, & Van Loosdrecht, 2015). A technology used in the AD process present-day is the high rate granular sludge technology which enables effective production of biogas from wastewater (Lettinga, van Velsen, Hobma, de Zeeuw, & Klapwijk, 1980). Key intermediates in the AD process are volatile fatty acids (VFA). These VFA can serve as a platform molecule, in the so-called carboxylate platform (Agler, Wrenn, Zinder, & Angenent, 2011; Holtzapple & Granda, 2009). VFA serves in this platform as a building block for the production of chemicals and polymers such as the production of polyhydroxyalkanoates (Marang, Jiang, van Loosdrecht, & Kleerebezem, 2013; Jelmer Tamis et al., 2018). Organic waste, of unknown composition and complexity can be degraded to these platform molecules using a fermentation process. In order to acquire VFA from an AD process, methanogenesis should be prevented. Methanogenesis is the conversion of (in)organic C1 and C2 intermediates to methane and carbon dioxide containing biogas. This process can be stopped by inhibiting the methanogens by e.g. working at low solid retention times (SRT), and/or working at a low pH with high VFA concentrations (Kleerebezem et al., 2015). Combining the granular sludge technology and the inhibition of methanogens, the principles of the AD process using granules can be applied to produce VFA from complex substrates.
Fermentation processes to yield VFA are usually conducted in continuous stirred tank reactors (CSTR) fed with glucose (Rombouts, Mos, Weissbrodt, Kleerebezem, & Van Loosdrecht, 2019; Temudo, Muyzer, Kleerebezem, & Van Loosdrecht, 2008). A drawback of using a chemostat type process is that low volumetric productivities are achieved and that all biomass produced is present in the effluent (J. Tamis, Joosse, van Loosdrecht, & Kleerebezem, 2015). Another type of operation would be the usage of granular sludge, an established technology in the AD process through the development of the Upflow Anaerobic Sludge Bed (UASB) reactor or more recently the ‘Nereda®’ technology applied in municipal wastewater treatment (Lettinga et al., 1980; Pronk et al., 2015). Using granular sludge technology the SRT can be uncoupled from the hydraulic retention time (HRT), resulting in high-rate systems (J. Tamis et al., 2015). Additionally, a lower solid content can be realized in the effluent using granular sludge through biomass removal from the sludge bed. Low solid contents are beneficial in processes converting the VFA produced into higher-value non soluble products like bioplastics. However, the application of granular sludge technology for the production of VFA is just touched upon and granulation and system control need to be persistent. In general, anaerobic granular sludge could serve as a platform to produce on an industrial scale VFA from organic wastewaters.
Even though VFA production from organic waste using anaerobic granular sludge has been demonstrated, many research questions remain (Jelmer Tamis et al., 2018). For example, the factors determining the product spectrum of the fermentation of glucose and other carbohydrates remain unclear. For non-granular processes it has been demonstrated that at higher pH values (6-8) the product spectrum will mainly consist out of acetate and ethanol, whereas at a lower pH (5-6) an acetate and butyrate mixture will be obtained (De Kok, Meijer, Van Loosdrecht, & Kleerebezem, 2013; Temudo, Kleerebezem, & van Loosdrecht, 2007). At lower pH-values, carbohydrate fermentations that produce organic acids often generate H2-gas as a by-product. Even though in some studies H2 is considered the main product of the fermentation process (Das & Veziroglu, 2008; Hallenbeck & Ghosh, 2009), H2 production lowers the overall VFA product yield. The produced H2 contains electrons originating from the substrate and the H2 leaves the bioreactor via the off-gas, resulting in a decrease of the VFA yield on substrate. Lastly, also biomass production can be considered as an unwanted side product of the fermentation process. In anaerobic processes where no (strong) electron acceptor is present the biomass yield is relatively low compared to aerobic processes. Still, in order to maximize VFA production the biomass yield should be minimized. Increasing the SRT (and therewith lowering the biomass specific growth rate) may result in higher VFA yields because the biomass yield is reduced due to higher substrate demands for maintenance purposes. Increasing the SRT can lead to a higher VFA yield as shown in previous studies (Bengtsson, Hallquist, Werker, & Welander, 2008; Bolaji & Dionisi, 2017). Overall, the fermentation of organic waste is a complex process and the factors that determine the product spectrum are largely unknown.
In this study the effect the SRT has on the product yield and product spectrum of the fermentation of glucose to VFA was investigated. A sequenting batch reactor (SBR) operated at pH 5.5 was used in which pulse-wise glucose was fed to anaerobic granular sludge. The main process performance indicators were; the VFA yield, the product spectrum; the sludge volume index (SVI) and the microbial community structure as identified using 16s rRNA sequencing. Three operational SRT were chosen namely, 1-2 d SRT, 10-20 d SRT and uncontrolled SRT. Challenges for this procces will be granulation at different SRT, avoidance of methanogenesis at longer SRT and reduced VFA production yield due to production of H2.

Materials and methods